Hi kTink!
We will be at the Swan at the same time. I'm not 100% sure we'll stick with those dates, but it is a great time to be at WDW.
We've stayed at the Dolphin once, last year. Parking is $8 or $9 per day (you can come and go as much as you like in that time and you get free parking at the parks).
As for perks, the main thing we liked was the proximity to EPCOT and the Boardwalk. With all the dining at the hotels and these areas, we just couldn't eat a bad meal, really. Also, service is TOP notch, according to most, though we did find the park ticket agents at S/D very difficult to deal with. Beds and noise level were fantastic. And we were accomodated with every request, resulting in a fantastic view, even though we were conventioneers. Transportation (bus) was near perfect. Way better that our experience at BW and POFQ. Doormen, bellmen, housekeeping, and front desk, bartenders, waiters, ALL were great. Add the great deal on the room (I mean, you can barely get a moderate Disney hotel for $125) and it is a bargain to me. We'd love to stay at WL, but just can't justify the extra $$. We also used the on-site laundry, and it was super clean and worked perfectly.
I'm sure others will tell you about the 24 hr food court, the boats, the pools, 5 star food, and the spa, but we weren't able to use these much. This time will be a different story.
Oh, and DH loved the internet access, though I find it a bit pricey.
Have fun!!